1. Field of the Invention
The present invention relates to a light emitting device utilizing an organic compound, and more detailedly to a light emitting device, particularly an organic electroluminescent device (organic EL device), having excellent luminance, efficiency and drive durability by doping a light emitting layer with plural compounds.
2. Related Background Art
The organic EL device is being actively investigated for its applications as a light emitting device capable of showing a high speed response and a high efficiency. The basic configuration of such device is shown in FIGS. 1A, 1B and 1C (for example see. Macromol. Symp., 125, 1-48 (1997)).
As shown in FIGS. 1A, 1B and 1C, the organic EL device is generally composed, on a transparent substrate 15, of a transparent electrode 14, a metal electrode 11, and an organic layer sandwiched therebetween and consisting of plural organic films.
In the configuration shown in FIG. 1A, the organic layer consists of a light emitting layer 12 and a hole transport layer 13. The transparent electrode 14 is composed for example of ITO having a large work function, thereby achieving satisfactory hall injection characteristics from the transparent electrode 14 into the hole transport layer 13. The metal electrode 11 is composed of a metallic material of a small work function such as aluminum, magnesium or an alloy thereof for achieving satisfactory electron injection characteristics into the light emitting layer 12. These electrodes have a film thickness of 50 to 200 nm.
In the light emitting layer 12, there is employed for example an aluminum quinolinol complex having electron transporting property and light emitting characteristics (as exemplified by Alq3 shown in the following). Also in the hole transport layer 13, there is employed a material showing electron donating property such as a biphenyl diamine derivative (as exemplified by α-NPD shown in the following).
The device of the above-described configuration shows an electric rectifying property, and, when an electric field is applied in such a manner that the metal electrode 11 becomes a cathode and the transparent electrode 14 becomes an anode, the electrons are injected from the metal electrode 11 into the light emitting layer 12 and the holes are injected from the transparent electrode 14 into the light emitting layer 12 through the hole transport layer 13.
The injected holes and electrons cause recombination in the light emitting layer 12 to generate excitons, thereby generating light emission. In this operation, the hole transport layer 13 serves as an electron blocking layer, whereby the efficiency of recombination is increased at the interface of the light emitting layer 12 and the hole transport layer 13 thereby improving the light emitting efficiency.
In the configuration shown in FIG. 1B, an electron transport layer 16 is provided between the metal electrode 11 and the light emitting layer 12 in FIG. 1A. Such configuration separates the light emission from the transportation of electrons and holes, thereby achieving more efficient carrier blocking and realizing efficient light emission. As the electron transport layer 16, there can be employed, for example, an oxadiazole derivative.
Conventionally, the light emission in the organic EL device is generally based on the fluorescence of molecules of a high emission center in a shift from a singlet exciton state to a base state. On the other hand, there is being investigated a device utilizing phosphorescence through a triplet exciton state, instead of the fluorescence through the singlet exciton state. Representative examples of the references reporting such device are:    1) D. F. O'Brien et al, Improved Energy Transfer In Electrophosphorescent Device, Applied Physics Letters Vol. 74, No. 3, p. 422 (1999), and    2) M. A. Baldo et al, Very High-efficiency Green Organic Light-emitting Devices Based On Electrophosphorescence, Applied Physics Letters, Vol. 75, No. 1, p. 4 (1999).
In these references, there is principally employed an organic layer of a 4-layered configuration as shown in FIG. 1C, consisting of a hole transport layer 13, a light emitting layer 12, an exciton diffusion preventing layer 17 and an electron transport layer 16 from the anode side. There are employed following carrier transporting materials and phosphorescence emitting materials, which are abbreviated as follows:
Alq3: aluminum-quinolinol complex
α-NPD: N4,N4′-dinaphthalen-1-yl-N4,N4′-dipheny-biphenyl-4,4′-diamine
CBP: 4,4′-N,N′-dicarbazole-biphenyl
BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline
PtOEP: platinum-octaethylporphilline complex
Ir(ppy)3: iridium-phenylpyridine complex

Also Forrest et al., Nature, 403, p. 750 discloses an EL device of laminated structure utilizing CBP as a host material of the light emitting layer, and causing triplet-singlet energy transfer from a green light emitting layer based on Ir(ppy))3 to a red light emitting layer based on DCM (dicyanomethylene).
These configurations are different from that of the present invention in that the co-existing light emitting materials have distant light emitting wavelengths and that the forming method does not involve vacuum evaporation of a mixture, as will be clarified later in the examples.
In the above-described organic EL device utilizing phosphorescent light emission, it is important to inject a larger amount of carriers into the light emitting layer at a lower voltage while maintaining the balance of electrons and positive holes at such lower voltage, in order to achieve a high luminance and a high efficiency.
Among such phosphorescent materials, there are known ones with low charge injecting and charge transporting properties, in which it is difficult to cause a large current at a low voltage.
Also many organic materials are known to form a cluster of plural molecules at the evaporation, and the light emitting layer involving such clusters is considered to show a locally high concentration of the light emitting material, leading to a loss in the light emitting efficiency of the device.
Also the organic materials are known to cause deterioration of the characteristics, for example by crystallization of the same molecules in the light emitting layer.
Because of the above-described background, there is desired a light emitting device capable of providing a high luminance of light emission and a long service life.